In known wastewater treatment methods, relying on the use of active sludge, suspension of the active sludge in a mixture with wastewater being treated is maintained by mixing of the contents of process chambers. Then the mixture is fed to a secondary sedimentation tank, where the sludge is deposited, and the decanted liquid, being purified wastewater, is fed to a collecting tank. The precipitated deposit is again used in the treatment process. In such processes, the concentration of the active sludge in treatment chambers is maintained at a level of 3.5 kg/m3, and this results in a considerable treatment volume for the respective plant.
From the Polish patent application, 172080 a flow wastewater treatment method is described, where the purification process is carried out consecutively within a wastewater dephosphatation chamber, wastewater denitrification chamber, and then a wastewater nitrification chamber, with the use of a return internal repetitive recirculation of nitrified wastewater from the aerated nitrification chamber for denitrification step. As a result of which, active sludge is subjected several times to alternating denitrification and nitrification processes.
Active sludge is in contact with all kinds of carbon present in the wastewater. The active sludge is more prone to use the available carbon forms. Therefore, in the effluent, there remain forms of carbon that are not suitable for life. Flow velocity through the individual chambers is high, and, and in the initial purification chambers the forms of carbon are readily available and abundant and are unrestrictedly accessible for the sludge. Sewage effluent from aeration chambers is carried to a suction chamber of a vacuum degassing tower. The effluent is then subject to vacuum degassing and routed to a secondary sedimentation tank. Deposits precipitated in the sedimentation tank are reused in the purification process and routed to the beginning of the purification process. This is done as the purification process maintains its capability to purify newly fed amounts of wastewater to the beginning of the process. Decanted liquid, being purified wastewater, is routed to a collection tank.
In this process, purification may occur at the actual sludge concentration of 6.5 to 7.5 kg of dry mass/m3. With a considerable decrease in cubic capacity of purification chambers and an increase of process intensity with regard to the so-far existing practical opportunities, this allows achieving a new higher level of purification as well as provides new opportunities for its technical development.
Typically, a phenomenon of intense denitrification occurs in active sludge chambers, and in the secondary sedimentation tank. Although nitrates are present, the denitrification process runs slower and its intensity is limited by absence of readily available carbon. Readily available carbon is an indispensable substrate of biochemical denitrification reaction. This reaction consists of decomposition of nitrates into oxygen and nitric acid. This reaction is subject to dissolution in gaseous nitrogen-undersaturated water, mixing, caused by vacuum degassing, the active sludge in wastewater being purified. But, this reaction does not result in denitrification-induced floatation and swelling of the sludge, and does not hinder thickening of the sludge on the bottom of the sedimentation tank. It is a reason for slow course of the denitrification reaction. The denitrification reaction is necessary and desired for the operation of the sedimentation tank when the nitrogen being produced is dissolved in an undersaturated aqueous solution.
Populations grow, urbanization and industrialization of the world induce increases in the amount of wastewater carrying biogenic contamination, particularly nitrogen, and causing harmful effects for the natural environment, such as intensive destruction by entrophisation, consisting of water algae blooming in surface water where the discharge for purified wastewater occurs. Prevention of this phenomenon requires use of complex methods of wastewater treatment, that may be highly expensive and operationally unreliable, and many involve considerable difficulties in operational handling for strict requirements for discharge of such entrophisation impurities.
With regard to the above, the solution according to the Polish patent application P380965, is an additional step is added to the secondary sedimentation tank, consisting of performing within the volume thereof, an additional intense and fast running process for wastewater treatment, resulting in removal of nitrogen compounds by means of denitrification.
According to application P380965, wastewater is mixed with active sludge and is purified in biological chambers. The sludge is then fed to a suction chamber of a vacuum degassing tower, where the mixture is subjected to vacuum degassing, followed by feeding to the secondary sedimentation tank and re-use of deposits collected in the secondary sedimentation tank for secondary purification of wastewater by removing nitrogen compounds. Then, in the volume of the secondary sedimentation tank, optionally at its inflow, at least one carbon compound is introduced, in at least one carbon compound such that comprises carbon readily available for active sludge. Intensive secondary denitrification is induced that causes decomposition of nitrates and removal of nitrogen from wastewater and continues until depletion of the readily available carbon. Simultaneously, the level of discharge of nitrogen from wastewater is increased to a value that allows for absorption of N2-undersaturated liquid, activated in the induced intense denitrification process of gaseous nitrogen, obtained from the use of readily available carbon. The nitrogen discharge comprises gaseous nitrogen released as a result of denitrification from the residual process with depletion of the source of readily available carbon. Moreover the level of undersaturation of the liquid with gaseous nitrogen is maintained so as to carry on sedimentation process under conditions that enable denitrification-induced hindrance of sedimentation capacity of the sludge.